On my prototype PCB, I have a microcontroller with reserved pins for UART communication with a debug terminal. I'm curious from a theory perspective about why my terminal works fine whether or not I connect ground to my USB UART adapter. I come from a computer science background, so assume my EE fundamentals are lacking.

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    \$\begingroup\$ Could be a signal wire(s) providing enough conductivity to GND to satisfy the other signal wire(s) etc etc \$\endgroup\$ – Harvard Aug 17 '16 at 20:41

Check whether your connector's shields (E.G. MicroUSB) are not doing the job of linking the grounds of the systems. Depending on how the USB to UART converter board you're using was built, the GND might be connected through the connector shielding and along the cable shielding. I'd start by measuring continuity between system's grounds with everything powered off.

  • \$\begingroup\$ I should have mentioned this in the question - there is no continuity on ground, the target is battery powered \$\endgroup\$ – Jacob Jennings Aug 17 '16 at 20:03
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    \$\begingroup\$ Aha, continuity was provided through the usb common ground from a debug adapter. \$\endgroup\$ – Jacob Jennings Aug 17 '16 at 21:00

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